When bolts are being put in place, it is often necessary to set the tensioning force in a defined manner. This can be done, for example, by means of a torque wrench. Since, as a rule, there is a relationship between the torque and the tensioning force, it is possible to ascertain the current tension on the basis of the current torque.
However, the tension can also be determined directly, whereby a so-called “direct tension indicator” (DTI) can be employed. Such a direct tension indicator is disclosed, for example, in German patent specification DE 103 16 632 B4. The disk disclosed in DE 103 16 632 B4 has an indicator for checking the tension that has been set on the screw bolt, whereby the indicator consists of capsules that are filled with a dye. At a predefined axial load, the dye comes out of the indicator and/or becomes visible at the edge of the indicator. In comparison to the indirect determination via the torque, the direct determination of the tension by means of a direct tension indicator is more precise since the relationship between the torque and the tension can be subject to fluctuations.
However, it has been found that direct tension indicators do not function reliably in certain cases when they are employed for concrete anchors.